Synopsis It is well established that protein synthesis by isolated chloroplast ribosomes is inhibited by the same antibiotics which inhibit protein synthesis by prokaryote ribosomes. The best studied example is chloramphenicol, which inhibits protein synthesis by isolated chloroplast from all the species so far tested; inhibition is shown only by the D-threo isomer of chloramphenicol, and not by the other stereo isomers. Protein synthesis by cytoplasmic ribosomes from plants is not inhibited by any of these antibiotics. On the other hand, cytoplasmic ribosomes are inhibited by cycloheximide, which does not affect the activity of chloroplast ribosomes.